8/3/2019 Conf Paper 2012

1/12

1

Proceedings of TFMS 2012

National Conference on Thermal, Fluid and Manufacturing Science

January 2021, 2012

Surat, Gujarat, India

Combustion Control with Trapped Residual Gas

(TRG) for 4-Stroke HCCI/CAI EnginesSantosh B Trimbake

Assistant Professor

Department of Mechanical Engineering

College of Military Engineering Pune Maharashtra 411031

santoshtrimbake @ yahoo.co.in

Mobile No: +919960431466

Abstract: Over the last decade, an alternative combustion

technology, commonly known as homogeneous charge compression

ignition (HCCI) or controlled auto-ignition (CAI) combustion, has

emerged that has the potential to achieve efficiencies in excess of

GDI engines and approaching those of current CI engines, with very

low NOx emissions and virtually no smoke emissions. While thepotential benefits of this new combustion technology are significant,

this combustion mode faces its own set of challenges, such as

difficulty in controlling the combustion phasing, a restricted

operating range, and high hydrocarbon emissions.

The most successful and practical approach to HCCI/CAI

combustion initiation and control in a gasoline engine is through the

use of large amounts of burned gases by trapping them within the

cylinder or through internal recirculation, as their thermal energy

will heat the charge to reach auto-ignition temperature and help totame the heat release rate.

This paper presents the review of combustion control

technology with residual gas trapping using variable valve actuation

for 4 stroke HCCI/CAI engine.

Advancements in combustion control with residual gas

trapping technology has been reviewed as reported in the literature,

as it can realized in both single/multi-cylinder production type PFI

(Port Fuel Injection) and DI (Direct Fuel Injection) gasoline engines

using the NVO (negative valve overlap) approach.

Initially the various aspects of residual gas trapping for

PFI gasoline engine have been presented. Subsequently, DI which

offers more independent control over CAI output and combustion

phasing, has been discussed. Further how fuel injection timing canbe used as an effective means to control CAI combustion has been

evaluated.

Keywords: CAI, HCCI, PFI, DI, Control, NVO & Residual

gas trapping

1 INTRODUCTION: HCCI is an alternative piston-engine

combustion process that can provide efficiencies as high as CI,

engines while, unlike CI engines, producing ultra-low NOx and PM

emissions. HCCI engines operate on the principle of having a dilute,

premixed charge that reacts and burns volumetrically throughout the

cylinder as it is compressed by the piston. In some regards, HCCI

incorporates the best features of both SI and CI. Most engines

employing HCCI to date have dual mode combustion systems inwhich traditional SI or CI combustion is used for operating

conditions where HCCI operation is more difficult. Typically, the

engine is cold-started as an SI or CI engine, then switched to HCCI

mode for low- to mid-load operation to obtain the benefits of HCCI

in this regime, which comprises a large portion of typical automotive

driving cycles. For high-load operation, the engine would again be

switched to SI or CIDI operation. Research efforts are underway to

extend the range of HCCI operation .Combustion control is the

biggest challenge to HCCI engines becoming a commercial success.

Trapped Residual gas method (TRG) with variable valve actuation

(VVA) seems to be one of the most effective and practical

combustion control approach for four-stroke gasoline engines(PFI/DI)

2 FUNDAMENTALS OF CAI/HCCI GASOLINE

ENGINES: CAI combustion is achieved by controlling the

temperature, pressure and composition of the air/fuel mixture so that

auto-ignited combustion can start at the right time and will proceed

without causing a runaway heat release rate. There is no direct

control over the ignition timing as in a SI or CI engine In an ideal

case, CAI/HCCI combustion can be described as controlled auto

ignition of a premixed fuel/air mixture and involves the simultaneous

reactive envelopment of the entire fuel/air mixture without a flame

front. As shown fig1, the initiation of combustion always occurs at

multiple sites in the premixed fuel/air mixture. The heat release

process is much faster than the conventional SI combustion and is

more closely described by a constant volume heat addition process,

This combustion mode also results in a more uniform and repeatable

heat release incomparison with that of SI operation. The cumulative

heat release in such an engine is therefore the sum of the heat

released Q from the complete mixture in the cylinder, m, each

combustion reaction, dq i and where k is the total number of heat

release reactions, and q is the heat released from the i th heat release

reaction involving per unit mass of fuel and air mixture (fig 2).

mailto:santoshtrimbake@yahoo.co.inmailto:santoshtrimbake@yahoo.co.inmailto:santoshtrimbake@yahoo.co.in

8/3/2019 Conf Paper 2012

2/12

2

2.1 Requirements For HCCI: The HCCI combustion process puts two

major requirements on the conditions in the cylinder:

(a) The temperature after compression stroke should equal the auto

ignition temperature of the fuel/air mixture.

(b) The mixture should be diluted enough to give reasonable burn

rate.

Fig 1 HCCI combustion process

Fig 2 heat released in HCCI combustion

3 CHALLENGES FOR HCCI COMBUSTION [1]: HCCI

combustion is achieved by controlling the temperature, pressure, and

composition of the fuel and air mixture so that it spontaneously

ignites in the engine. This control system is fundamentally morechallenging than using a spark plug or fuel injector to determine

ignition timing as used in SI and CI engines, respectively. The recent

advent of electronic engine controls has enabled consideration of

HCCI combustion for application to commercial engines. Even so,

several technical barriers must be overcome to make HCCI engines

applicable to a wide range of vehicles and viable for high volume

production. Significant challenges include:

Controlling Ignition Timing and Burn Rate Over a Rangeof Engine Speeds and Loads

Extending the Operating Range of HCCI to High EngineLoads

Cold-Starts and transient response with HCCI Engines

Minimizing Hydrocarbon and Carbon Monoxide Emissions4 ADVANCEMENTS IN COMBUSTION CONTROL

TECHNOLOGIES [1]: Combustion control is the biggest

challenge to HCCI engines becoming a commercial success. For this

reason, several methods have been proposed for achieving HCCI

engine control over the wide range of operating conditions required

for typical transportation-engine applications. Control technologies

reported in the literature have demonstrated some degree of success.

Some of the proposed methods include:

Trapped Residual Gas (TRG) using VVA(Variable valve actuation): Here residual gas from the

previous cycle is trapped for next cycle by VVA and used

as driving force for charge temperature to attained auto

ignition temperature Amount and temperature of TRG is

the tool to control the combustion.

Variable compression ratio (VCR): VCR engine hasthe potential to achieve satisfactory operation in HCCImode over a wide range of conditions because the

compression ratio can be adjusted as the operating

conditions change.

Thermal control In this methodology, thermal energyfrom exhaust gas recirculation (EGR) and compression

work from a supercharger are either recycled or rejected to

obtain satisfactory combustion.

Ignition-enhancing additives HCCI engine controlcould be achieved by using two fuels with different octane

ratings.

5. PRINCIPLE OF CAI/HCCI OPERATION WITH

RESIDUAL GAS TRAPPING: The principle of CAI operationwith residual gas trapping is to initiate CAI combustion and to

control the subsequent heat release rate by trapping large and variable

amounts of residual gases in the cylinder. The burned gases from the

previous cycle are trapped in the cylinder by closing the exhaust

valves relatively early. Unlike spark ignition operation, the engine

load is controlled primarily by the exhaust valve timing/lift. As the

load is decreased, the exhaust valve closure (EVC) is advanced so

that more burned gases are trapped, and the intake valve opening

(IVO) is retarded accordingly to avoid backflows of residuals into the

intake ports. This leads to negative valve overlap, NVO (fig 3)

which is in contrast to positive valve overlap normally used in SI

engines in order to maximize the volumetric efficiency Combustion

control by retained residual gas is often called controlled auto-

ignition, CAI. Fig 4 shows how residual gas can be retained using a

NVO and fig 5 shows the valve timing diagram for residual gas

trapping .

The larger amount of residuals trapped will lead to less

fresh charge being admitted into the cylinder and hence less fuel

being burned. Conversely, retarded EVC will lead to reduced amount

of residuals and hence more fresh charge to be flowed into the

cylinder for greater work output. It is important to note that such

operations are carried out with wide open throttle and hence there are

no pumping losses associated with a partly closed throttle as in the SI

operation.

From above discussion it can be realized that Variablevalve actuation (VVA) needs to be used to control the timing / lift

of exhaust valve and inlet valve to control the initial charge

temperature by retaining variable amount of residual gas.

VVA exists in many flavors with different degrees of

freedom. VVA could be implemented in an engine with mechanical,

magnetic, or hydraulic valve actuators. There are systems that merely

provide cam phasing on the intake/exhaust cam shaft. Other systems

provide cam profile switching, CPS, or combinations of cam phasing

and CPS. Finally there are systems that provide fully variable valve

8/3/2019 Conf Paper 2012

3/12

3

timing as well as lift (electro-hydraulic / electro-magnetic VVA

system).

The NVO approach is attractive since many production

engines have VVA capability that allows CPS and cam phasing.

Fig 3 Illustration of NVO strategy (dashed) as opposed to the

normal positive valve overlap strategy (solid).

Fig 4 In-cylinder P- diagram with residual gas trapping

Fig 5 Valve timing diagrams for different operation strategies.

6. CAI OPERATION IN A FOUR-STROKE PORT FUEL

INJECTION (PFI) GASOLINE ENGINE

J. Li & et. al. [2] first investigated the performance and emission

characteristic on 4-cylinder production type PFI gasoline engine

using substantially standard components, modified only in cam

dimensions to control the gas exchange process in order tosignificantly increase the trapped residuals. operating with CAI and

equipped with Variable Cam Timing (VCT) .The engine used was a

Ford 1.7 Litre Zetec SE 16-valve 4-cylinder PFI gasoline engine with

sequential fuel injection strategy to ensure that the same mixture

preparation event was applied to each of the four cylinders The intake

and exhaust camshafts were equipped with two independent VCT

system with a pair of special camshafts of reduced lift and the

compression ratio was kept at 10.3. The fuel used was the standard

unleaded gasoline of RON 95. During the tests the throttle was kept

at wide open and the air flow was changed by varying the cam

timings, which could be continuously changed by up to 40 degrees

crank angle. All experiments were carried out when the coolant

reached 90 0 C or over in order to minimize the effects of coolant

temperature.

6.1Performance and combustion characteristics

It was found that the largely increased trapped residuals alone were

sufficient to achieve CAI in this engine and with VCT, a range of

loads between 0.5 and 4 bar BMEP and engine speeds between 1000and 3500 rpm ( fig 6) were mapped for CAI fuel consumption and

exhaust emissions. The measured CAI results were compared with

those of Spark Ignition (SI) combustion in the same engine but with

standard camshafts at the same speeds and loads. There was a linear

correlation between the residual fraction and engine output,

independent of the engine speed (fig 7). The higher the residual

fraction was, the lower the torque became. As the engine was

operated at WOT, the mass in the cylinder was more or less the same

and only the mixture concentration changed .Fig 8 shows variation

of the maximum rate of pressure rise with residuals or load. In most

cases, the maximum rate of pressure rise decreased with residuals or

increased with load, so was the peak cylinder pressure. The rate of

maximum pressure rise varied between 1 and 7 bar/CA.

Fig 6 CAI operation range with residual gas trapping

6.2Fuel consumption and emission characteristics

Fig 9 to12 compare the fuel consumption and emission results of the

CAI combustion mode and SI mode from the same engine. The

comparison showed more than 30% reduction in BSFC (Fig 9) is

seen . Up to 99% reduction in NOx at low loads(fig 10) is seen with

CAI operation But it should be noted that CAI combustion in the 4-

stroke gasoline engine had been always associated with higher CO

emissions than the SI combustion until the residual gas trapping

method was employed.(fig 12). Fig 11 shows that the unburned HCs

were much higher from CAI combustion than that from SI

combustion with port-fuel injection, but they were on a par with those

from the stratified charge direct injection gasoline engine

7 EFFECT OF DI ON CAI COMBUSTION IN THE 4-

STROKE GASOLINE ENGINE During the CAI operation,

engine output is principally controlled by the EVC timing. As the

engine load or speed increases, combustion starts earlier and burns

faster, leading to too rapid a rate of heat release and very high peak

cylinder pressure as well as higher fuel consumption.

8/3/2019 Conf Paper 2012

4/12

4

Fig 7 Effect of residual fraction on maxi. Rate of Pressure rise.

Fig 8 Relationship between residual fraction & engine o/p

Fig 9 Changes in BSFC (%) with CAI combustion relative to SI.

Fig 10 Changes in BSNOx emissions (%) with CAI combustion

At the low load region, the very retarded combustion

causes large cyclic variation and even partial burn therefore, it will be

desirable and necessary to find other means capable of more

independent control over the combustion phasing from the engine

load, in order to improve the CAI combustion and its operational

range Research done at the Brunel Universitys laboratory [35] has

shown that direct fuel injection into the cylinder can be used as one

of the most effective means of controlling the combustion phasing for

optimised engine performance and emissions. This section will

present some of the main findings from such studies

Fig 11 Changes in BSHCs emissions (%) with CAI combustion

relative to SI.

Fig 12 Changes in BSCO emissions (%) with CAI combustion

relative to SI.

There are three significant phases occurring sequentially in the CAI

engine cycle with negative valve overlap, namely, residuals

trapping, residuals conditioning and CAI combustion. Each phase

has an effect on the following, with the end-of-cycle conditions

feeding back to the first phase in order to sustain the CAI operation

continuously.

The residual trapping phase is controlled primarily by the exhaustvalve closing timing (variable early EVC) and the trapped residuals

temperature Once trapped, the mass of the residuals is fixed for the

subsequent cycle, but its temperature and pressure are variable during

theresiduals conditioning phase according to re-compression and re-

expansion and further heat subtraction and heat addition if fuel can be

injected directly into the residuals during that period. At the end of

the residuals conditioning phase which also marks the beginning the

intake period (variable late IVO), the released temperature and

pressure of the residuals will affect the intake air flow, the charge

dilution (residuals/total volume ratio) and the combustible charge

temperature, while direct fuel injection during the intake and

compression periods will affect the charge homogeneity or

stratification as well as charge temperature and quantity, leading up

to the CAI combustion phase which is characterized by the CAI

ignition timing, heat release rate, IMEP, ISFC and exhaust emissions.

The final exhaust gas temperature after CAI is then fed back to the

next cycle to initiate the next residuals trapping phase

Initially single fuel injection timing strategy has been used to study

its effect on combustion characteristics which has been further

grouped into three categories [7] :

( i ) Early injections during the negative valve overlap period, in

which fuel is injected into the hot residual gas in the cylinder for the

8/3/2019 Conf Paper 2012

5/12

5

purpose of reforming the fuel or initiating the minor combustion if

possible, and improving ignitability;

(ii) Mid-injections during the intake stroke and early compression

stroke to create a homogeneous mixture of different charge

temperature or quantity;

(iii) Late injections during the late compression stroke for

charge/thermal stratification.

7.1Early injections during the NVO period R Standing & et al.[ 5 ] Investigated the effects of injection timing and valve timings on

CAI operation in a multi-cylinder DI gasoline engine having

displacement volume of 1.6 liters and a compression ratio of 11.5. A

high pressure swirl injector mounted below and between the two

intake valves was operated at an injection pressure of 100 bars. Both

intake and exhaust camshafts were equipped with VVT devices and

their cam lobes were machined to produce maximum valve lifts

around 2 mm. fig13 shows the effect of fuel injection timings on the

main combustion process. It can be seen that the earlier the fuel

injection took place during the negative valve overlap period, the

more advanced the start of main combustion when the engine

operated with a lean fuel air mixture. However, in the case of

stoichiometric mixture shown in fig 14 fuel injection at 20 BTDCduring the recompression phase led to the earliest combustion,

followed by 40 BTDC and TDC injections. Delayed injection into the

intake stroke resulted in the most retarded combustion and the lowest

peak pressure in both stoichiometric and lean mixtures. In addition it

can be seen that the early injections into lean burned mixtures led to

more advanced combustion than for stoichiometric mixtures.

In order to understand better the underlying mechanisms, detailed

analysis has been carried out at Brunel University [6] on the physical

and chemical processes taking place within the cylinder by means of

3-D full cycle engine simulation. The simulation programme is based

on the KIVA3v with improvements in turbulence, the gas/wall heat

transfer, spray atomization, ignition and combustion and it takes intoaccount the gas exchange processes that are crucial to the residual gas

trapping method. The Shell ignition model was chosen and has been

modified to simulate the auto-ignition process in low temperature

combustion. For the high temperature combustion, a characteristic

time combustion model is used. The transition from auto-ignition to

the main combustion process is based on the local cell temperature:

when the temperature of a cell exceeds 1080K, high temperature

combustion model is activated for such a cell. The simulation

programme was validated against engine experiments before it was

applied to study CAI combustions.

The main combustion characteristics for injections during

the NVO period are summarised in Table 1. The value of net IMEP is

closely related to combustion phasing and pumping loss. Both too

early combustion phasing and higher pumping losses contribute to

lower IMEP values with injections at 40 and 20 ATDCoverlap, as

compared with the injection at 75 ATDCoverlap. Comparing the

cases with injections at 75 ATDCoverlap and TDCoverlap, the

combustion phasings of those two injection cases are quite similar,

however the higher pumping losses result in lower IMEP with

injection at TDCoverlap

7.2 Mid and late injections during the intake and compression

strokes Using the same 3-D full cycle engine simulation, the effect

and underlying mechanism of mid and late injections on CAI

combustion were examined Figure 15 shows the pressure and heat

release rate varying with injection timings. Comparing the two

injections during the intake stroke, the start of combustion is slightly

retarded with later injection timing(SOI at 150 ATDC Overlap), leading

to lower peak pressure. Table.2 shows that there is little difference in

the compression temperature between the two injections during the

intake stroke. The delayed start of combustion with later injection is

therefore likely related to the time available for fuel to mix with air

and subsequent low temperature chemical reactions. However, the

combustion phasing is advanced, as the injection is retarded further

into the compression stroke (SOI at 218 ATDC Overlap). This is more

likely due to the in-cylinder mixture stratification.

Based on the above studies, the mechanisms of combustion

phasing control by injection timing in a lean-burn CAI DI gasoline

engine can be summarized herewith. The factors include the

thermal/chemical effects caused by early injection during the NVO

period, or charge cooling effect by injection during the intake stroke,

or fuel stratification effect by late injection at the compression stroke.

Heat release or thermal effect associated with injection during the

NVO period has a dominant effect on advancing the start of maincombustion. The chemical effect is secondary and its presence

promotes the first stage of ignition during the compression stroke.

However, injection during the negative valve overlap period can also

slow down the main combustion process, if the in-cylinder

temperature during the recompression process is reduced

significantly due to chargecoolingeffect and hence less or no heat

release reactions can take place during the recompression and re-

expansion. The late injection during the compression stroke can lead

to an advanced combustion due to charge stratification, whilst the

injection during the intake stroke slows down the start of main

combustion by charge cooling effects.

Fig 13 In cylinder pressure and heat release with various injection

timing for lean mixtures

Fig14 In cylinder pressure and heat release with various injection

timing for stoichiometric mixtures

8/3/2019 Conf Paper 2012

6/12

6

Table 1 Effect of early fuel injection timing on CAI combustion at

1500 rpm & = 1.2

Fig 15 Pressure & heat rate profiles with mid & late injections

Table 2 Effect of mid & late fuel injection timing on CAI combustion

at 1500 rpm & = 1.2

CONCLUSION: CAI combustion control has been realized in

both single / multi-cylinder production type PFI and DI gasoline

engines using the NVO approach with VVA technology that allows

CPS and cam phasing. It is achieved by trapping copious amount of

burned gases in the cylinder through EVC early. Such engines are

characterized with superior fuel economy due to the lack of throttling

loss and extremely low NOx emissions. However, the range of CAI

operation needs to be significantly extended not only to the high load

region but also the low load region to take advantage the fuel

economy and low emission benefits across the vehicle driving modes.

DI is one such technology which offers more independent control

over CAI output and combustion phasing for optimized performance

and emissions as compared to PFI. It has shown that fuel injection

timing in DI can be used as an effective means to control CAI

combustion.

REFERENCES

1. Homogeneous Charge Compression Ignition(HCCI) Technology ,

A Report by Office of Transportation Technologies ,U.S.

Department of Energy Efficiency and Renewable Energy, 2001.

2. Li, J., Zhao, H., and Ladommatos, N., Research and development

of controlled auto-ignition (CAI) combustion in a four-stroke multi-

cylinder gasoline engine, SAE paper 2001-01-3608, 2001.3. Leach, B., Zhao, H., Li, Y., Ma, T., Control of CAI combustionthrough injection timing in a GDI engine with an air-assited injector,SAE Paper 2005-01-0134, 2005.

4. Li, Y., Zhao H., Bruzos N., Ma T., and Leach B., Effect ofInjection Timing on Mixture and CAI Combustion in a GDI Engine

with an Air-Assisted Injector, SAE Paper 2006-01-0206, SAESpecial Publication SP-2005, 2006.

5. Standing, R., Kalian, N., Ma, T., Zhao, H., Effects of injectiontiming and valve timings on CAI operation in a multi-cylinder DI

gasoline engine, SAE paper 2005- 01-0132, 2005.

6. Cao, L, Zhao, H., Jiang. X, Kalin, N., Investigation into the Effect

of Injection Timing on Stoichiometric and Lean CAI operations in a

4-Stroke GDI Engine, SAE Paper 2006-01-0417, 2006.7. Hua Zhao HCCI and CAI engines for the automotive industry

1st ed., Woodhead Publishing Limited ,ISBN 978-1-84569-128-8,2007.

Authors brief bio-data and photograph

Shri Santosh B. Trimbake presently Assistant professor in

Mechanical Engg Dept of College of Military Engineering Pune

411031 affiliated to JNU, New Delhi. Author is M Tech (Thermal &

Fluids Engg) from IIT Powai. Area of interest is I C Engines &

Refrigeration & Air Conditioning. He Possesses total experience of

14 years, out of which 5 years served in Defence Industry (Ordnance

factory) & remaining 9 years in teaching

*santoshtrimbake@yahoo.co.in

8/3/2019 Conf Paper 2012

7/12

7

8/3/2019 Conf Paper 2012

8/12

8

8/3/2019 Conf Paper 2012

9/12

9

8/3/2019 Conf Paper 2012

10/12

10

8/3/2019 Conf Paper 2012

11/12

11

8/3/2019 Conf Paper 2012

12/12

12